Unintentional edge and face selection
It may be a bug or your selection is wrong. I would strongly suggest upgrading to the latest stable release, which is 2.82a at this time. To get a finer selection, try Z "Wireframe". You can also toggle "X-Ray" with Alt+Z, and you can hide / unhide faces / edges / vertices by selecting them and then hitting H and Alt+H.

My method:
Select the faces you want to extrude with "X-Ray" turned off. Turn "X-Ray" back on to see if any faces got selected by accident. You can also Ctrl+i to invert the face selection. H to hide them. This will leave you with only the face you want to extrude, and you will know for sure if any extras got selected.

One last thing:
You might have overlapping edges causing those extra faces to form on extrude. Before extruding, try F3 Merge by Distance and adjust the Merge Distance to something small that only overlapping vertices will be affected by.